Abstract

Misool Island, 140 km southeast of Sorong (Irian Jaya), has an area of 3000 km² and a virgin interior. Earlier geological studies were mainly concerned with the Mesozoic biostratigraphy of the excellent coastal exposures. In 1970 Phillips Petroleum Company Indonesia spudded 3 unsuccessful offshore wells in the northeast. In 1981 a joint Indonesian-Australian team prepared a geologic map based on lithostratigraphic units. More recently; Pertamina has undertaken numerous traverses across the island. The detailed stratigraphic reconstruction allows better understanding of the geologic framework and might lead to a better evaluation of the hydrocarbon prospects.

The oldest strata consist of slightly metamorphosed clay, and greywacke of turbidite origin, with Triassic fossils in the upper parts (Ligu-Keskain Formation). The whole sequence was uplifted and eroded, and thereafter shallow water carbonates (Bogal Formation) with some fossiliferous marl (Lios Member) were deposited. After a period of non-deposition, marine sedimentation resumed in the Early Jurassic by deposition of basal conglomerate followed by dark grey, fine-grained clastics (Yefbie Formation), overlain in the Middle Jurassic by bedded marls and silts (Demu Formation), and then by dark grey, fine grained shales rich in fossils (Lelinta Formation). The Early Cretaceous was dominated by bioturbated, deep water, fine-grained lime mud with chert bands and nodules (Gamta and Waaf Formations). The Late Cretaceous was characterized by fine grained clastics rich in Inoceramus and Rudists (Fafanlap Formation). The latest stage of the Cretaceous was dominated by oolitic limestones of the Yabatano Member.

The Early Tertiary was initiated by porous, quartz-rich clastics (Daram Formation). The subsequent part of the Tertiary was dominated by oolitic to biostromal platform limestone with thin marl intercalations.

Misool was part of Irian Jaya's Bird's Head Block which drifted away during the Early Mesozoic. Maximum basin depth was reached during Gamta and Waaf deposition. A thick clastic wedge derived from the Bird's Head uplift was again deposited during the Late Cretaceous and Paleogene. Thereafter, under shallow marine conditions the block was pushed southwestward along the Sorong-Misool Fault, which resulted in a complicated structure.

The Mesozoic Yefbie, Lelinta and Fafanlap Formations are the most likely to have hydrocarbon generating potential. The Mesozoic Bogal and Demu Formations, and best of all, the Late Mesozoic-Early Tertiary Daram Formation, are the most promising reservoir rocks. The best Daram prospects seem to be in the area southeast of Misool.